Tuning of optical projection system to optimize image-edge placement

ABSTRACT

Method for minimization of degradation of images created by the projector tool turns on the optimization of the pattern-imaging by adjusting parameters and hardware of the projector to judiciously impact the placement of various image edges at different locations in the image field. Adjustments to the projector (exposure tool) include a change of a setup parameter of the exposure tool and/or scanning synchronization and/or a change of a signature of the optical system of the exposure tool determined as a result of minimizing the pre-determined cost function(s) that are parts of a comprehensive edge-placement error model.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority from and benefit of the U.S.provisional patent application Ser. No. 62/112,423 filed on Feb. 5, 2015and titled “Single Exposure Lithography Edge Placement Model”. Thepresent application also claims priority from and benefit of the USprovisional patent application Ser. No. 62/164,435 filed on May 20, 2015and titled “Tuning of Optical Projection System to Optimize Image-EdgePlacement”. The present application is also a continuation-in-part fromthe international application no. PCT/US2015/048037 filed on Sep. 2,2015 and titled “Pattern Edge Placement Predictor and Monitor forLithographic Exposure Tool”, which in turn claims priority from the USprovisional patent application No. 62/044,777, filed on Sep. 2, 2014 andtitled “Pattern Edge Placement Predictor and Monitor for LithographicExposure Tool”. The disclosure of each of the above-identified patentdocuments is incorporated by reference herein.

TECHNICAL FIELD

The present invention relates generally to modification of a hardware ofa lithographic exposure tool to vary the placement of image pattern(s)formed on a target substrate and, more particularly, to the process ofadjustment of element(s) of a projection optical system of the exposuretool to achieve precise locations of such patterns and the pattern edgesthat have been predefined as a result of optimization of the image-edgeplacement.

BACKGROUND

Photolithography is a process commonly used for defining features duringsemiconductor wafer processing, for example during the fabrication ofintegrated devices (and, in a specific case, integrated circuits orICs), micro electro-mechanical systems (MEMS), or microopto-electro-mechanical systems (MOEMS), collectively referred to hereinas integrated devices. Photolithography generally involves applying aphotoresist material (“resist”) to a wafer, irradiating the resist usingchosen radiation (for example, light) the spatial distribution of whichis appropriately patterned, then developing the patterned resist,etching a material of the wafer or depositing different materials on theparts of wafer exposed in the patterned resist, and, finally, removingthe remaining resist after etching or deposition of the materials. Inphotolithography, a critical dimension (CD) is a characteristic sizethat corresponds to various features critical to the integrated deviceperformance that need to be patterned on a chosen surface (such as thewafer surface, for example), e.g., a target feature width, length and/orspacing between features.

The process of integrated device fabrication involves multiple steps ofresist patterning followed by wafer etching or materials deposition.During such fabrication process, patterns are laid down on a reticle,the image of which is later formed with a photolithography tool on atarget substrate (such as a semiconductor wafer) in a sequence ofpatterning steps. The formation of an image of the reticle is followedby the process of formation (which may include the etch and/ordeposition processes) of features the shapes of which correspond to theshapes laid down on the reticle. Each pattering step results infabrication of a patterned layer of material that overlaps with someother layers fabricated at different steps of the integrated devicefabrication process. As a result, integrated devices contain stacks oflayers with overlapping geometrical patterns formed in variousmaterials, and connected with the patterns in different layers of theseintegrated devices.

While the CD-control of lithographic patterns is an important aspect ofthe lithographic process used to ensure that the end product meets thedesign specification, related art overlooks the fact that performance ofthe integrated device components fabricated by lithographic exposure isdetermined not only by the shapes and CDs of the lithographic patterns,but also by mutual positioning of the edges of various imagepatterns—for example, by locations of the edges of a given image patternin a given layer of the integrated device relative to (or with respectto) those of patterns in various other layers of the integrated device.Empirical evidence suggests that precise determination and monitoring ofthe locations of pattern edges proves to be critical for accurate andprecise integrated device fabrication. Indeed, spatial misplacement ofan image pattern (that, otherwise, has correct shape and dimensions) ina chosen layer of the integrated device relative to image patterns inintegrated device layer(s) located above or below such chosen layerleads to degradation of operation of the integrated device or itsmalfunction. The yield of integrated device manufacture is determinednot only by how well and/or precisely the individual layers ofintegrated devices are patterned, but also by how well patterns invarious integrated device layers are aligned with respect to otherpatterns in other layers.

The time between the initial lithographic process development tomanufacturing implementation of the developed process has shrunkdramatically in recent years. In general, it is rarely feasible toexperimentally evaluate and optimize all aspects of a lithographicprocess prior to manufacturing introduction. It is well recognized thatability to predict and to monitor the pattern edges is critical tosuccessful control of integrated device manufacture (see, for example,Progler, Bukofsky, and Wheeler, “Method to Budget and Optimize TotalDevice Overlay”, SPIE Vol 3679, pages 193-207, 1999). What is morecritical, however, is the process of configuring the optical projectionsystem used for the formation of an image on the target substrate suchas to minimize the pattern overlay errors alluded to above. While therelated art addresses the image overlay procedure and describes patternedge placement with the use of the results of measurement of overlaymarks, the related art does not explain how to measure the spatialerrors of the pattern edge placement on a target substrate with respectto the ideal location(s) of such placement. The related art is alsosilent with respect to a projection optics system that would allow theimage overlay to be minimized or otherwise corrected (let alone how totune the existing projection system, used to create the very imagesparticipating in the image overlay) in order to minimize or correct theimage overlay. Furthermore, while emphasizing the device overlay budget,the related art does not address co-optimization of the pattern imagingand placement by adjusting entire imaging setup, including illuminationand aberration contents of the projection optics. The solutions to suchco-optimization and modification of the used-for-patterning projectionoptical systems are required.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood by referring to thefollowing Detailed Description in conjunction with the generallynot-to-scale Drawings, of which:

FIG. 1 is a diagram illustrating schematically an embodiment of alithographic exposure tool;

FIG. 2 schematically illustrates an example of operational environmentconfigured to implement an embodiment of a method of the invention;

FIGS. 3A, 3B, 3C illustrate static and dynamic image exposure. FIG. 3A:top view of image field statically exposed in one shot instep-and-repeat projectors. FIG. 3B: side view of dynamic imaging takingplace in scanning projectors. FIG. 3C: top view of dynamic exposurecarried out with simultaneously scanning the reticle and the wafer in adirection perpendicular to the optical axis of the lens used to form animage of the reticle. Dynamic exposure of images takes place whilesimultaneously scanning the reticle in XY plane, in X direction. Crossesin FIGS. 3A, 3C indicate the locations of marks tracking the imageoverlay.

FIGS. 4A, 4B present vector maps of image-pattern shifts caused byoverlay performance of a projector at various locations across apatterned wafer. Each red arrow 410 indicates an overlay-driven patternshift at a corresponding to such arrow location at the patterned wafer.FIG. 4A shows uncorrected pattern displacements, while FIG. 4B shows thepattern displacements after systematic overlay components have beencorrected. Such corrections are determined by first exposing a testwafer and analyzing and computing the systematic components of thepattern shifts on this test wafer, as FIG. 4A. These systematic shiftcomponents are then used to compute corrections for image placement,which corrections are applied during subsequent exposure of otherwafers, thereby leading to corrected overlay shown in FIG. 4B. As theresult, the subsequent wafers carry the corrected overlay distribution(the example of which is shown in FIG. 4B). Note: the correctionsdetermined based on Eqs. (2) through (4) can combine variouscontributions to edge displacements based on the choice of ω, α, and β.This choice is made by the user who conducts the edge-displacementoptimization based on the user's understanding on how various, overlay-and imaging-driven, contributions to edge displacement impact theperformance of the final integrated devices. For example, the user ofthe projector may conclude that the overlay-driven contributions aresmall in comparison with CD uniformity, and he/she might suppress ω andemphasize β. Under different circumstances, the user might conclude thatCD uniformity does not make large contribution to the edge displacement,so he might suppress β, emphasize α, and keep ω at a pre-determinedlevel. Such strategy should be based the knowledge of imagingrequirements and performance of any given pattern. It is understood,therefore, that the vectors 420 of FIG. 4B indicate the image-patternshift that remains not corrected.

FIGS. 5A, 5B illustrate an exposure (image) field, marked within thewafer 510 with the image field rectangular contours 514, and, for agiven field 520, identify thirty-five locations 530 across the field(placed in seven rows at five different locations) where overlay marks,such as those indicated by crosses 310 in FIG. 3A would be located.Black lines 514A represent design intent under perfect alignment of eachpattern exposure field, referred to as “shot”. FIG. 5A illustrates thealignment of the shot centers within the coordinate system of the wafer.The contours 514B, 514C show the exposure field locations determinedafter applying the across-wafer and intra-field corrections,respectively. FIG. 5B illustrates the projection of the shots around theshot center. The squares show the pattern locations and the arrows showpattern shifts. The black squares 530A illustrate the design intentpattern location. The squares 530B and arrows 530C show shifts resultingfrom correction of intra-field overlay errors. The contour 520B, square530D, and arrows 530E show the actual shifts caused by overlay random,uncorrectable components. The wafer, shot size, shot displacements, andshape excursions are shown not to scale.

FIGS. 6A, 6B, 6C, and 6D illustrate imaging drivers of pattern edgedisplacements. FIG. 6A illustrates the layout of the pattern 610representing a design-intent captured on the reticle pattern; the redcross 620 indicates the pattern target position. FIG. 5B illustrates theimpact of various imaging conditions that degrade image contrast,causing thereby a change in image dimensions from 610 to 630. FIG. 6Cillustrates, with a green arrow 640, a pattern displacement from 620 tothe shifted-image center position 650 (marked with a green cross) causedby a combined impact of projector signature and projection aberrations.FIG. 6D illustrates the impact of resist on the image edges (foradditional information, see PCT/US2015/048037). EPE_(Top), EPE_(Bottom),EPE_(Left), and EPE_(Right) are markings representing the locations ofthe resist pattern edges.

FIG. 7 and insert present a schematic diagram illustrating a patternedge displacement caused by a projector. The red arrow 710 and square714 show the actual placemen of the pattern after overlay systematicerrors have been corrected. These red arrow and square are the same ascorresponding element 530D, 530E in FIG. 5. The green arrow 640 showsshift caused by optical operation of the projector, i.e. shift caused bya combined impact of projector signature and projection aberrationsshifting the pattern image relative to the actual pattern placementtarget location. The shift marked by the green arrow 640 is the same asin graphs of FIGS. 6C and 6D. The shape and dimension change of thepattern imaged by the projector, shown as light oval 720, relative todesign intent marked as 610, illustrates a contribution to edgedisplacements caused by the pattern CD variations.

FIGS. 8A, 8B, 8C, 8D: Vector graphs illustrating errors in pattern-edgeplacement (the shifts of pattern edges). Each vector / arrow 810 in themap shows the direction and magnitude of the edge shift at one of thelocations on the wafer. FIGS. 8A, 8B illustrate maps of the edgeplacement errors for left and right vertical edges. FIGS. 8C, 8Dillustrate maps of the edge placement errors for lower and upperhorizontal edges. Each of inserts in the maps of FIGS. 8A through 8Drepresents an enlarged view of the corresponding exposure field in thewafer.

FIGS. 9A, 9B, and 9C illustrate edge shifts of horizontal line patternsat 33 locations across the image field. The arrows show intra-field edgeshifts such as the ones represented by the green arrow 640 in FIG. 6C.Here, the blue arrows 910 and the green arrows 914 show shifts of lowerand upper horizontal edges respectively. FIG. 9A shows uncorrected edgeshifts. FIG. 9B shows the edge distribution after corrections accordingto cost function of Eq. (2), optimizing the shape of the edge shiftdistribution. FIG. 9C shows the edge distribution after correctionsaccording to cost function of Eq. (3), minimizing the largest edge shiftin the image field. Corrections such as the ones that resulted in edgeshift distribution shown in FIG. 9C, reduced the largest shift occurringin the center of the lower row, marked by the green array 914A in FIG.9B. The shift scales in graphs of FIGS. 9A, 9B, and 9C are the same.

FIGS. 10A, 10B, 10C, 10D illustrate edge shifts images of dense contacthole patterns at 33 locations across the image field. The horizontalarrows show the shifts of vertical left and right edges (in this case,both edges shifted the same way). Vertical arrows show the shifts ofhorizontal lower and upper edges (both edges shifted the same way). FIG.10A shows the uncorrected shifts of contact hole patterns imaged withwindmill quadrupole illuminator. FIG. 10B shows the edge shifts afterthe intra-field corrections were applied. The scales of the arrow shiftsin FIGS. 10A, 10B are the same. FIG. 10C shows the same shifts as inFIG. 10B but with the scale magnified by 4× to show intra-fielddistribution of the shifts. FIG. 10D shows the edge shifts afterdifferent intra-field corrections were applied. The corrections thatresulted in the edge shifts shown in FIG. 10D were obtained by tuningthe illuminator from a windmill quadrupole layout shown in FIG. 11A tocross quadrupole layout, shown in in FIG. 11B. The arrow scales in FIGS.10C, 10D are the same, i.e. they are 4× of the arrow scales in FIGS.10A, 10B.

FIGS. 11A, 11B present distributions of light at the illuminator exitpupil, which illustrates properties of two settings of the illuminator,observed at one of the principle planes of the projector. Thedistribution of FIG. 11A corresponds to the case of a windmillilluminator, while the distribution of FIG. 11B corresponds to a crossquadrupole illuminator. The bright segment areas 1104 in the graphsrepresents the segments in the illuminator pupil from which light ispropagating towards the reticle. The grey arrows 1110 mark the azimuthalpolarization of the illumination radiation.

DETAILED DESCRIPTION

Embodiments of the present invention provide a method and system foradjustment of a hardware of an optical-projection based imaging system(such as a lithographic exposure system configured to form images on atarget substrate by projecting light thereon) based not only on theresults of monitoring and recording of locations of overlay markstracking the target locations of patterns, but also on the process ofoptimization of such results in light of (i) pattern-specificdisplacements of the patterns relative to target locations (representedby the overlay marks), and (ii) pattern-edge displacements caused by thevariations of CDs that occur during patterning process. Subject toadjustment are optical and/or geometrical parameters of the imagingsystem.

As a result, embodiments of the invention solve the problem ofoptimizing the feature edge placement and imaging at multiple locationsin the image field of patterns projected in lithographic equipment bymodifying the aberrations of (a) the illumination portion (or“illuminator”) of the overall optical-projection based imaging system,(b) the projection portion or projection lens (which terms is usedinterchangeably with “projections optics”) of the overalloptical-projection based imaging system, and/or (c) a scanning system ofthe overall optical-projection based imaging system to co-optimize theplacement of images and imaging performance based on a comprehensiveedge-placement consideration. The term “projector” may be used hereininterchangeably with the terms “optical-projection based imaging system”and “projection imaging system” to denote the overall system (such as alithographic exposure tool). Examples of aberrations to be defined andmodified include image field distortions and phase-front aberrationscomponents at the exit pupil of the projection lens.

Specifically, embodiments of the invention are configured to optimizethe pattern imaging process such as to minimize the degradation ofimages created by the projector. Such minimization of the degradation ofthe image, formed on the layer of a photoresist, is effectuated byadjusting the parameters and the setup of the projector to judiciouslyimpact the placement of various image edges at different locations inthe image field. Adjustments to the projector effectuated according tothe idea of the present invention include a change of a setup parameterof the exposure tool (such as, for example, optical axis tilt,distribution of illumination in a principal plane of the exposure tool,polarization status in a principal plane of the projection system,illuminator flare, numerical aperture of the projection lens, projectionlens aberrations, shape of the pupil of the projection lens, opticalapodization, optical transmission and flare of the projection lens)and/or scanning synchronization errors and/or a change of a signature ofthe optical system of the exposure tool. The required adjustments aredefined by minimizing the pre-determined cost function(s) (also referredto interchangeably as merit function(s)) that are parts of acomprehensive edge-placement error (EPE) model. The term “flare” is tobe understood as representing light scattered within a given opticalcomponent or a subsystem of the projector. The terms “polarizationstatus” or “polarization” refer to any of the following descriptors: adegree of polarization, a polarization direction (vector), andnon-polarized background, as well as to a combination of any of suchdescriptors. The errors in polarization (or polarization errors) definea deviation of the polarization status from the target polarizationstatus.

As is well known in the art, the operation of the projection systemunfolds in repeated steps. At each step, one image of the reticle isprojected on the wafer (or, to be more specific, on a layer ofphotoresist carried by the wafer). Imaging of the reticle patterns canbe done in a static manner, by exposing the entire image field all atonce, or in a dynamic manner, by scanning a section of the reticlethrough an image slit of the projection system. For each reticle, thisprocess is repeated multiple times to form multiple images of eachpatterned layer on the wafer. After one layer has been patterned andfabricated, another reticle may be used to pattern and fabricate anotherlayer of an integrated device. The operation of the projection systemaims at placing the images at various locations in the image plane (theplane at which the images of the reticle are formed). Such operationrequires precise alignment of the image field (a two-dimensional spacecontaining the images of the reticle) within the frame of referenceestablished in the image plane. The alignment operation of the projectoris hardly perfect. Therefore, the placement (locations) of the images isdegraded by various detractors (caused by the imperfect performance ofvarious subsystems of the projector), which include, for example, thequality of alignment of the image plane placement sensors, theinterpretation of signals such sensors produce, the imperfect operationof the image plane placement controllers, and fluctuations and drifts inthe imaging environment. The placement inaccuracies resulting fromimperfect overlay operation lead to displacements of the image fieldsrelative to the frame of reference in the image plane.

In addition, a projection system used to form the images is not perfectin a sense that it does not produce identical images perfectly matchingspecifications at every location of the image field. Pattern imagesprojected at the image plane are subject to various distortions of shapeand displacements with respect to their target locations (that arespecified by the pattern design). The origins of these displacements caninclude the mechanical operation of the projection system (for example,the image field distortions occurring during the scanning operation of ascanner). The distortions and displacements can also be caused by theoptical characteristics of the projection system (for example,aberrations of the illuminator and/or projection optics) caused byimperfect design and manufacturing tolerances of the optic involved, andin particular, by illuminator imperfections, image field distortions,image field curvature, and residual aberrations represented by phasefront variation of the projection lens pupil, to name just a few.

As the result, during the operation of the projector, the placement ofthe images of various patterns at the target locations in the imageplane is affected by at least two features: 1) a patterning overlay orimage overlay performance, which impacts the ability of the projector toalign the images relative to their target positions, and 2) imagedisplacements caused by the optical characteristics of the projectionsystem.

A complete description of the image displacements, caused by the overlayand the imaging operation of the projectors, requires a completion ofthe task not yet addressed by related art—that the image edge placementerrors be quantified. The capture of edge-placement errors impacts theprojector overlay and the imaging performance at various locationsacross the image plane.

Non-limiting Example of a Photolithographic (Exposure) Apparatus andRelated Systems.

FIG. 1 is a schematic view illustrating but one implementation of aphotolithography apparatus 100 (e.g., scanner, imaging system, exposureapparatus, etc.) in accordance with the present invention. The waferpositioning unit 152 includes a wafer stage 151, the following stage103A and an actuator 106. The wafer stage 151 comprises a wafer chuckthat holds a wafer W and an interferometer minor IM. The exposureapparatus 100 can also include an encoder to measure stage position (notshown for simplicity of illustration). The base 101 may be supported bya plurality of isolators 154 (sometimes referred to a reaction frame), aleast one of which may include a gimbal air bearing. The following stagebase 103A is supported by a wafer stage frame (reaction frame) 166. Theadditional actuator 106 is supported on the ground G through a reactionframe. The wafer positioning stage 152 is configured to move the waferstage 151 with multiple (e.g., three to six) degrees of freedom underthe precision control provided by an optionally-programmable drivecontrol unit and/or system controller 162, and to position and orientthe wafer W as desired relative to the projection optics 146. In thespecific embodiment 100, the wafer stage 151 has six degrees of freedomand utilizes forces vectored in the Z direction and generated by thex-motor and the y-motor of the wafer positioning stage 152 to control aleveling of the wafer W. However, a wafer table having three degrees offreedom (such as for example Z, θ_(X), θ_(Y)) or six degrees of freedomcan be attached to the wafer stage 151 to control the leveling of thewafer. The wafer table may include the wafer chuck, at least three voicecoil motors (not shown), and a bearing system. The wafer table may belevitated in the vertical plane by the voice coil motors and supportedon the wafer stage 151 by the bearing system so that the wafer table canmove relative to the wafer stage 151.

The reaction force generated by the wafer stage 151 motion in the Xdirection can be canceled by motion of the base 101 and the additionalactuator 106. Further, the reaction force generated by the wafer stagemotion in the Y direction can be canceled by the motion of the followingstage base 103A.

An illumination lens or illuminator 142 is supported by a frame 172. Theillumination lens 142 projects radiant energy (e.g., light) through amask pattern on a reticle R that is supported by and scanned using areticle stage RS. (Alternatively, in the case of when the system 100utilizes extreme ultraviolet (EUV) radiation, the reticle R may beconfigured to operate in reflection). The reticle stage RS may have areticle coarse stage for coarse motion and a reticle fine stage for finemotion. In this case, the reticle coarse stage corresponds to thetranslation stage table 110, with one degree of freedom. The reactionforce generated by the motion of the reticle stage RS can bemechanically released to the ground through a reticle stage frame andthe isolator 154 (in one example—in accordance with the structuresdescribed in JP Hei 8-330224 and U.S. Pat. No. 5,874,820, the entirecontents of each of which are incorporated by reference herein). Thelight is focused by a projection optics (in one embodiment—a lensassembly) 146 supported on a projection optics frame 150 and released tothe ground through isolator 154. The assembly 146 may includeradiation-transmitting glass elements (refractive lements), reflectors(such as mirrors) or a combination of the two (thereby forming acatadioptric projection optics).

An interferometer 156 is supported on the projection optics frame 150and configured to detect the position of the wafer stage 151 and outputthe information of the position of the wafer stage 151 to the systemcontroller 162. A second interferometer 158 is supported on theprojection optics frame 150 and configured to detect the position of thereticle stage and to produce an output containing the information of theposition to the system controller. The system controller controls adrive control unit to position the reticle R at a desired position andorientation relative to the wafer W and/or the projection optics 146.

There are numerous different types of photolithographic devices whichcan benefit from employing an embodiment of the present invention. Forexample, the apparatus 100 may comprise an exposure apparatus that canbe used as a scanning type photolithography system, which exposes thepattern from reticle R onto wafer W with reticle R and wafer W movingsynchronously. In a scanning type lithographic device, reticle R ismoved perpendicular to an optical axis of projection optics 146 byreticle stage and wafer W is moved perpendicular to an optical axis ofprojection optics 146 by wafer positioning stage 152 Scanning of reticleR and wafer W occurs while reticle R and wafer W are movingsynchronously but in opposite directions along mutually parallel axesparallel to the x-axis.

Alternatively, the exposure apparatus 100 can be a step-and-repeat typephotolithography system that exposes reticle R, while reticle R andwafer W are stationary. In the step and repeat process, wafer W is in afixed position relative to reticle R and projection optics 146 duringthe exposure of an individual field. Subsequently, between consecutiveexposure steps, wafer W is consecutively moved by wafer positioningstage 152 perpendicular to the optical axis of projection optics 146 sothat the next field of semiconductor wafer W is brought into positionrelative to projection optics 146 and reticle R for exposure. Followingthis process, the images on reticle R are sequentially exposed onto thefields of wafer W so that the next field of semiconductor wafer W isbrought into position relative to projection optics 146 and reticle R.

The use of the exposure apparatus 100 schematically presented in FIG. 1is generally not limited to a photolithography system for semiconductormanufacturing. The apparatus 100 (an exposure apparatus or tool), can beused for example as an LCD photolithography system that exposes a liquidcrystal display device pattern onto a rectangular glass plate or aphotolithography system for manufacturing a thin film magnetic head.

In the illumination system 142, the illumination source can be a sourceconfigured to generate light at g-line (436 nm), i-line (365 nm), or toinclude a KrF excimer laser (248 nm), ArF excimer laser (193 nm), F2laser (157 nm) or to generate radiation in EUV (for example, at about13.5 nm).

With respect to projection optics 146, when far ultra-violet rays suchas the excimer laser is used, glass materials such as quartz andfluorite that transmit far ultra-violet rays are preferably used. Whenthe F2 type laser, projection optics 146 should preferably be eithercatadioptric or refractive (a reticle should also preferably be areflective type). When extreme ultra-violet (EUV) rays or x-rays areused the projection optics 46 should preferably be fully reflective, asshould the reticle.

With an exposure device that employs vacuum ultra-violet radiation (VUV)of wavelength 200 nm or shorter, use of the catadioptric type opticalsystem can be considered. Examples of the catadioptric type of opticalsystem include the disclosure Japan Patent Application Disclosure No.8-171054 published in the Official Gazette for Laid-Open PatentApplications and its counterpart U.S. Pat. No. 5,668,672, as well asJapanese Patent Application Disclosure No. 10-20195 and its counterpartU.S. Pat. No. 5,835,275. In these cases, the reflecting optical devicecan be a catadioptric optical system incorporating a beam splitter andconcave mirror. Japanese Patent Application Disclosure No. 8-334695published in the Official Gazette for Laid-Open Patent Applications andits counterpart U.S. Pat. No. 5,689,377 as well as Japanese PatentApplication Disclosure No. 10-3039 and its counterpart U.S. Pat. No.5,892,117 also use a reflecting-refracting type of optical systemincorporating a concave minor, etc., but without a beam splitter, andcan also be employed with this invention. The disclosure of each of theabove-mentioned U.S. patents, as well as the Japanese patentapplications published in the Office Gazette for Laid-Open PatentApplications is incorporated herein by reference.

Further, in photolithography systems, when linear motors that differfrom the motors shown in the above embodiments (see U.S. Pat. No.5,623,853 or U.S. Pat. No. 5,528,118) are used in one of a wafer stageor a reticle stage, the linear motors can be either an air levitationtype employing air bearings or a magnetic levitation type using Lorentzforce or reactance force. Additionally, the stage could move along aguide, or it could be a guideless type stage that uses no guide. Thedisclosure of each of U.S. Pat. NO. 5,623,853 and U.S. Pat. No.5,528,118 is incorporated herein by reference.

Alternatively, one of the stages could be driven by a planar motor,which drives the stage by electromagnetic force generated by a magnetunit having two-dimensionally arranged magnets and an armature coil unithaving two-dimensionally arranged coils in facing positions. With thistype of driving system, either one of the magnet unit and the armaturecoil unit is connected to the stage, and the other unit is mounted onthe moving plane side of the stage.

Movement of the stages as described above generates reaction forces thatcan affect performance of the photolithography system. Reaction forcesgenerated by the wafer (substrate) stage motion can be mechanicallyreleased to the floor (ground) by use of a frame member as described inU.S. Pat. No. 5,528,118 and published Japanese Patent ApplicationDisclosure No. 8-166475. Additionally, reaction forces generated by thereticle (mask) stage motion can be mechanically released to the floor(ground) by use of a frame member as described in U.S. Pat. NO.5,874,820 and published Japanese Patent Application Disclosure No.8-330224. The disclosure of each of U.S. Pat. No. 5,528,118 and U.S.Pat. No. 5,874,820 and Japanese Patent Application Disclosure No.8-330224 is incorporated herein by reference.

FIG. 2 provides a schematic illustration of environment 210 configuredto manage the processes effectuated by the system 100. The environment210 includes a server 212 that can perform the processes describedherein using appropriately structured computer program code(s). Asshould be appreciated by those of skill in the art, the server 212includes a computing device 214 having one or more processors 220,memory 222, an input-output (I/O) interface 224, and a bus 226. Thememory 222 can include local memory employed during actual execution ofprogram code(s), as one non-limiting example. The server 212 and/orcomputing device 214 are configured to read and/or receive informationfrom the scanner 100, and use this information to predict criticaldimensions of the images pattern and placement of its edges across thewafer that is being processed in the scanner 100. As used herein, theterms scanner and scanner apparatus refer to a photolithographyapparatus (e.g., imaging system, exposure apparatus, etc.) used inlithography.

The one or more processors 220 may be dedicated processors programmedfor execution of particular processes or combination of processes inaccordance with the invention, which may be performed on the server 212and/or the computing device 214. The server 212 and/or computing device214 may also be dedicated to particular processes or combination ofprocesses in accordance with the invention. Accordingly, the computingdevice 214 and/or server 212 can include any combination of generaland/or specific purpose hardware (e.g., one or more electronic circuitssuch as dedicated processors 220) and/or computer program code(s). Theserver 212 and/or computing device 214 are configured to communicateover any type of communications link, such as, for example: wired and/orwireless links; any combination of one or more types of networks (e.g.,the Internet, a wide area network, a local area network, a virtualprivate network, etc.); and/or utilize any combination of transmissiontechniques and protocols.

The computing device also includes an I/O device 228 that may beexternal to either the computing device 214 or the server 212. The I/Odevice 228 can be, for example, a device that is configured to enable anindividual (user) to interact with the computing device 214, such as adisplay equipped with GUI. In embodiments, the user can enterinformation into the system by way of the GUI (I/O device 228). In oneexample, the input items can be accessible to the user by a dialog box.In addition, it is contemplated that the I/O device 228 is configured tolead the user through the input requirements by providing input boxesfor textual input or pointer action.

By way of illustration, the I/O device 228 is configured to accept dataassociated with the imaging apparatus 100 and reticle/mask R of FIG. 1,amongst other information. Such data can include, for example,user-defined laser wavelength, laser bandwidth, laser spectrum,immersion and dry exposure data, a default index of refraction (forwater), pupil intensity, immersion exposure, threshold information(e.g., low intensity information from pupilgram files), polarizationinformation, etc. The mask information may include, for example, editingcapabilities for amplitude and phase information, etc., as well asaccepting GDS or OASIS mask files]. The server 212 (and/or computingdevice 214) includes a centralized device repository, e.g., tangible,non-transitory storage memory system 230. In embodiments, thecentralized device repository 230 is configured and/or designed to storethe computer code and library information (data).

According to the idea of the invention, the locations of the patternedges are assessed at various points or positions across the wafer andwithin the exposure field while images of the reticle are being formed(projected by the imaging apparatus, be it a scanner or a staticstepper) , by

-   -   acquiring data representing        -   i. repeatability and/or accuracy of placement of image(s)            onto the resist layer (which is independent from defocus and            dose of exposure traces), affecting, for example, aspects of            wafer alignment accuracy and repeatability, determined by            the wafer and wafer stage operational conditions, both on            the scale of the whole wafer and on the scale of a given            image field (also referred to as shot); and/or        -   ii. optical imaging conditions and parameters (such as            descriptors of the illumination and projection setup,            determining pattern-specific image displacements; and/or        -   iii. imaging tool performance such as defocus and dose of            exposure captured by the traces that are generated by the            scanner, that affect the quality and accuracy and precision            of optical image formation on the wafer resist layer; and    -   modifying at least one of mechanical and optical characteristic        of the projection system based on these data to form a        transformed projection system to minimize the image edge        placement errors.

In a specific embodiment, when the imaging apparatus includes a scanner,the locations of the image edges are assessed in situ, contemporaneouslywith the wafer being processed in the scanner.

Image Placement Driver(s) or Causes of Image-Edge Displacements.

During the exposure process, the purpose of the operation of theprojector alignment system is to ensure that various exposure fields aredelivered to (placed at) the target locations of corresponding imagefields (“shots”).

Schematics of FIGS. 3A, 3B, 3C provide illustrations to static anddynamic image exposure. FIG. 3A provides a top view of an image fieldstatically exposed in one shot in step-and-repeat projectors. FIG. 3Billustrates a side view of dynamic imaging taking place in scanningprojectors. FIG. 3C presents a top view of a dynamic exposure carriedout by simultaneously scanning the reticle and the wafer in a directionperpendicular to the optical axis of the projection lens used to form animage of the reticle. Dynamic exposure of images takes place whilesimultaneously scanning the reticle in XY plane, in X direction. Crossesin FIGS. 3A, 3C indicate the locations of marks tracking the imageoverlay.

The capability of the projector to place the shots on targets isreferred to as “overlay”. The overlay performance of the projector islimited by the ability of the system to accurately align the shots withthe target locations. Some of the deviations between the actualplacements of shots and the target locations are repeatable and can bequantified and corrected, while other small, residual overlaydisplacements remain uncorrected (and can be random or systematic). Theoverlay performance of the projection system is tested prior to orduring the wafer exposure to correct the repeatable deviations such thatafter the appropriate corrections are introduced, the overlaysubstantially consists of small, uncorrected displacements caused byresidual errors in overlay performance of the projector. The initial,uncorrected results of image-pattern shifts caused by overlayperformance and those corrected as a result of application of theembodiment of the invention are discussed below in reference to FIGS.4A, 4B.

Imaging Driver(s) or Optical-Imaging Causes of Image-Edge Displacements

It is appreciated that imaging conditions may differ at variouslocations in the exposure field. These differences may be due tovariations in imaging conditions such as mask pattern CD variation(s),reticle bending, illumination layout and polarization variations, thenumerical aperture (NA) of the projection lens, aberration andapodization variations, image field curvature, scanner flare, andlocalized resist responses that are induced by the resist process.

The detailed discussion of the errors in edge placement of the finalimage of the reticle caused by issues related to the variations of theCD of the mask (reticle) pattern was presented, for example, inPCT/US2015/048037. In comparison, this disclosure addresses imagingcauses associated with variation of imaging conditions that areunrelated to the mask pattern. The distortion of the image field, theimage field curvature, illuminator and projection lens aberrations,various displacements and/or changes in alignment of components of theoptical projection system with respect to the design provide examples ofsuch factors that cause variations of image placement and dimensions atvarious locations across the image field and that are not related to themask pattern. These image-dimension variations and image displacements,induced by the process of image formation in the projector and combinedwith the image displacement caused by the degradation in overlayperformance, result in displacements of pattern edges at variouslocations in the image field.

To this end, FIGS. 5A, 5B illustrate an exposure (image) field, markedwithin the wafer 510 with the image field rectangular contours 514, and,for a given field 520, identify thirty-five locations 530 across thefield (placed in seven rows at five different locations) where overlaymarks, such as those indicated by crosses 310 in FIG. 3A would belocated. Black lines 514A represent design intent under perfectalignment of each pattern exposure field, referred to as “shot”. FIG. 5Aillustrates the alignment of the shot centers within the coordinatesystem of the wafer. The blue and magenta contours 514B, 514C show theexposure field locations determined after applying the across-wafer andintra-field corrections, respectively. FIG. 5B illustrates theprojection of the shots around the shot center. The squares show thepattern locations and the arrows show pattern shifts. The black squares530A illustrate the design intent pattern location. The magenta squares530B and arrows 530C show shifts resulting from correction ofintra-field overlay errors. The red contour 520B, square 530D, andarrows 530E show the actual shifts caused by overlay random,uncorrectable components. The wafer, shot size, shot displacements, andshape excursions are shown not to scale.

FIGS. 6A, 6B, 6C, and 6D provide illustration to optical-imaging driversof pattern edge displacements. FIG. 6A shows the layout of the pattern610 representing a design-intent captured on the reticle pattern; thered cross 620 indicates the pattern target position. FIG. 5B illustratesthe impact of various imaging conditions that degrade image contrast,causing thereby a change in image dimensions from 610 to 630. FIG. 6Cillustrates, with a green arrow 640, a pattern displacement from 620 tothe shifted-image center position 650 (marked with a green cross) causedby a combined impact of projector signature and projection aberrations.FIG. 6D illustrates the impact of resist on the image edges (foradditional information, see PCT/US2015/048037). EP E_(Top), EPE_(Bottom), EP E_(Left), and EP E_(Right) are markings representing thelocations of the resist pattern edges.

FIG. 7 and insert present a schematic diagram illustrating a patternedge displacement caused by a projector. The red arrow 710 and square714 show the actual placemen of the pattern after overlay systematicerrors have been corrected. These red arrow and square are the same ascorresponding element 530D, 530E in FIG. 5. The green arrow 640 showsshift caused by optical operation of the projector, i.e. shift caused bya combined impact of projector signature and projection aberrationsshifting the pattern image relative to the actual pattern placementtarget location. The shift marked by the green arrow 640 is the same asin graphs of FIGS. 6C and 6D. The shape and dimension change of thepattern imaged by the projector, shown as light oval 720, relative todesign intent marked as 610, illustrates a contribution to edgedisplacements caused by the pattern CD variations.

FIGS. 8A, 8B, 8C, 8D provide vector graphs illustrating errors inpattern-edge placement (the shifts of pattern edges). Each vector /arrow 810 in the map shows the direction and magnitude of the edge shiftat one of the locations on the wafer. FIGS. 8A, 8B illustrate maps ofthe edge placement errors for left and right vertical edges. FIGS. 8C,8D illustrate maps of the edge placement errors for lower and upperhorizontal edges. Each of inserts in the maps of FIGS. 8A through 8Drepresents an enlarged view of the corresponding exposure field in thewafer.

Representation of Edge-Placement Errors.

All of the above causes in the image dimensions and image placementslead to image edge placement errors, EPEs. Indexing multiple locationsin each image field by 1 and multiple edge of a given image by i , theoverall edge placement error EPE i,l for the placement of the i-th edgeof a given image at the l-th location is quantified by the sum:

EPE _(i,l) =EPE _(i,l) ^(OVL) +EPE _(i,l) ^(POS) +EPE _(i,l) ^(IMG)  (1)

where

-   -   EPE_(i,l) ^(OVL) is the contribution to i-th image edge        placement error due to image displacement caused by the        projection overlay at the 1-th location. This contribution to        the edge placement error is caused by system errors other than        those occurring in the optical sub-systems of the projection        system, except for distortion errors. It is typically determined        from an overlay error measurement. This contributions to edge        displacements are independent of the image dimensions.    -   EPE_(i.l) ^(IMG) is the contribution to the i-th image edge        placement error due to image size (CD) variations at the l-th        location. In the related art, the CD variation is measured, as        discussed in detail in Ser. No. 62/112,423 but is not accounted        towards the EPE. These contributions to edge displacements are        dependent on the pattern dimensions.    -   Finally, EPE_(i,l) ^(POS) is the contribution to the i-th image        edge placement error due to image displacement caused by imaging        at the l-th location. According to the idea of this invention,        the value of this contribution to the image-edge placement error        is assessed from the sensitivity of the pattern placement to the        imaging system setup and imaging conditions.

In practice, determination or assessment of the value of suchcontribution is effectuated either by a computation of the imagingperformance of the system or via imaging tests.

Cost Functions as a Measure of Edge Displacements.

Cost functions, representing edge placement errors of various patternimages at various image locations, can be expressed as:

C _(dist)=Σ_(i,l)(ω_(i,l) *EPE _(i,l) ^(OVL)+α_(i,l) *EPE _(i,l)^(POS)+β_(i,l)*EPE_(i,l) ^(IMG))^(k) ^(i,l)   (2),

c _(Max)=Max for Any i,l|ω _(i,l) *EPE _(i,l) ^(OVL)+α_(i,l) *EPE _(i,l)^(POS) +β _(i,l) *EPE _(i,l) ^(IMG)|  (3)

and

c _(Rng)=Max for Any i,l (ω_(i,l) *EPE _(i,l) ^(OVL)+α_(i,l) *EPE _(i,l)^(POS)+β_(i,l) *EPE _(i,l) ^(IMG))−Min for Any i,l(ω_(i,l) *EPE _(i,l)^(OVL)+α_(i,l) *EPE _(i,l) ^(POS)+β_(i,l) *EPE _(i,l) ^(IMG))   (4)

Here, Eq. (2) establishes a metric representing the distribution of theedge placement error. It should be used when the patterning processrequires control of the pattern edge placement distribution of apopulation of selected image edges. Stated differently, the costfunction expressed by Eq. (2) is applicable when statisticaldistribution of a large number of image-edges at various locations haveto be optimized.

Eq. (3) establishes a metric representing the maximum of the edgeplacement error, determined over the population of selected edges. Itshould be used when patterning process requires control over the maximumdisplacement of the pattern edge within the edge population.

Eq. (4) establishes a metric representing the range of the edgeplacement errors over the populaton of selected edges. It should be usedwhen the pattering process requires control over the range of thepattern edge distribution. Generally, cost functions expressed by Eqs.(3) and (4) are applicable when the population of edges is sparse andthe imaging performance is limited by a few pattern edges.

In Eqs. (2), (3), and (4):

-   -   indices i and l run over the population of chosen edges of        images at select locations. Accordingly, different sets of i and        l indices establishes the cost functions defined by the choice        of edges of images at various locations across the image field;    -   ω_(i,l), α_(i,l), β_(i,l) and k_(i,l) are location- and        edge-specific parameters selected to emphasize the performance        of chosen edges at desired locations. These parameters represent        “weights” assigned to emphasize chosen edges at chosen locations        over other edges in various locations, to achieve flexibility in        scanner adjustments with respect to a variety of edge        displacements if diverse patterns, some of which depending on        particularities may be more critical to the device performance        than the others. These parameters can be the same for each edge        and location in formulae (2) through (4), or they can be        different for various subsets of edges and locations. The choice        of these parameters determines a degree to which various pattern        edges at the corresponding locations impact the final cost of        the optimization. The choice of ω_(i,l), α_(i,l), β_(i,l) and        k_(i,l) should be made based on the evaluation of effect of the        statistics of the population of edges on the final performance        of the integrated device design. For example: a particular cost        function might include ships of left, right, upper and lower        edges of a pattern, while the most critical for the design of        the integrated device is the placement of upper edges in the        center of the image field. To address this situation, ω_(i,l),        α_(i,l), β_(i,l) and k_(i,l) for the 1-th location in the field        center and ith upper edge should have a higher value than the        weights corresponding to the rest of the edges at other        locations;    -   the terms (Max for Any i, l) and (Min for Any i, l) stand,        respectively, for maximum and minimum “over the population of        i-th edges and l-th locations”.

FIGS. 4A, 4B provide illustrations of the overlay performance of aprojector by showing vector maps of image-pattern shifts caused byimperfection of such performance at various positions across a patternedwafer. Each arrow 410 indicates an overlay-driven pattern shift at acorresponding to such arrow location at the patterned wafer.

FIG. 4A shows uncorrected pattern displacements, while FIG. 4B shows thepattern displacements after systematic overlay components have beencorrected. Such corrections are determined by first exposing a testwafer and analyzing and computing the systematic components of thepattern shifts on this test wafer, as FIG. 4A. These systematic shiftcomponents are then used to compute corrections for image placement,which corrections are applied during subsequent exposure of otherwafers, thereby leading to corrected overlay shown in FIG. 4B. As theresult, the subsequent wafers carry the corrected overlay distribution(the example of which is shown in FIG. 4B). Notably, the correctionsdetermined based on Eqs. (2) through (4) can combine variouscontributions to edge displacements based on the choice of ω, α, and β.This choice is made by the user who conducts the edge-displacementoptimization based on the user's understanding on how various, overlay-and imaging-driven, contributions to edge displacement impact theperformance of the final integrated devices. For example, the user ofthe projector may conclude that the overlay-driven contributions aresmall in comparison with CD uniformity, and he/she might suppress ω andemphasize β. Under different circumstances, the user might conclude thatCD uniformity does not make large contribution to the edge displacement,so he might suppress β, emphasize α, and keep ω at a pre-determinedlevel. Such strategy should be based the knowledge of imagingrequirements and performance of any given pattern. It is understood,therefore, that the vectors 420 of FIG. 4B indicate the image-patternshift that remains not corrected.

Tuning Of Projection Optics to Optimize Image-Edge Displacement

Changes to the field curvature of the image field of the projectionlens, changes in image field distortions and/or projection lensaberrations cause the changes in placement and shapes of images atvarious locations in the image field. Therefore, by adjusting theprojection lens parameters that affect the projection lens image fieldcurvature, image field distortions and/or the aberrations of theprojection lens, the cost-function Eqs. (2) through (4) can beoptimized, leading therefore to optimization in image-edge placementerror populations. The projection lens adjustments that minimize costfunctions expressed by Eqs. (2) through (4) cause optimization of thereticle pattern imaging by the projection lens.

Tuning Of Projector Setup to Optimize Pattern Edge Displacements

Modifications of the projection optical system that lead to theoptimization of the cost functions (2) through (4) are highly dependenton the particular imaging setup and projector signatures representingilluminator and projection lens characteristics. Therefore, the optimalsolutions are, in practice, functions of various scanner attributes, orsignatures, representing scanner conditions and setup.

-   -   The projector signatures include characteristics such as, for        example, optical axis tilts, illuminator flare and polarization        errors, shape of the pupil of the projection lens pupil, optical        apodization, optical transmission and flare of the projection        optics, and scanning synchronization errors.    -   The projector setup (or projector imaging setup), on the other        hand, defines parameters of the illuminator such as, for        example, illumination distribution and polarization status, or        the numerical aperture of the projection lens.

According to the idea of the invention, the projector signatures andimaging setup are tuned with the purpose of minimizing the costfunctions (such as those expressed by Eqs. 2 through 4, for example) tooptimize the across-the-image-field image edge placement errors.

FIGS. 9A, 9B, 9C, 10A, 10B, 10C, 10D, 11A, and 11B provide examples ofoperation of the embodiment of the invention, leading to the optimizededge placement distributions. Here,

-   -   the edge distribution optimum shown in FIG. 9B was obtained by        minimizing cost function of Eq. (4). The cost function of Eq.        (4), representing the range of the edge shifts, for edge        distribution in FIG. 9B was reduced by 12% relative to the cost        function of Eq. (4) for edge distribution of FIG. 9A. At the        same time, the maximum edge displacement, represented by cost        function of Eq. (3) in FIG. 9B increased by 8% relative to the        cost maximum edge displacement shown in FIG. 9A. The projector        tuning that resulted in edge placement improvement shown in FIG.        9B involved image field shift of 0.35 nm and tuning of the image        field distortion via field magnification in y-direction by 0.015        parts per million and field rotation by −0.24 parts per million;    -   the edge distribution optimum shown in FIG. 9C was obtained by        minimizing cost function of Eq. (3). The cost function of Eq.        (3), representing the maximum edge displacement, for edge        distribution of FIG. 9C was reduced by 3% relative to the cost        function of Eq. (3) for the edge distribution of FIG. 9A, and it        was reduced by 11% relative to the cost function of Eq. (3) for        the distribution of edges shown in FIG. 9B. The projector tuning        that resulted in improvement of edge placement shown in FIG. 9C        involved shifting the image field by 0.05 nm, and tuning of the        image field distortion via field magnification in y direction by        0.015 parts per million and field rotation by −0.24 parts per        million;    -   the edge distribution optimum shown in FIGS. 10B and 10C was        obtained by minimizing cost function of Eq. (2). The cost        function of Eq. (2), representing distribution standard        deviation of edges in FIGS. 10B and 10C, was reduced by 61% for        vertical edges and by 7% for horizontal edges, relative to the        cost function of Eq. (2) for distribution of the corresponding        edges in FIG. 10A. The projector tuning that resulted in        improvement of the edge placement shown in FIGS. 10B and 10C        involved tuning of the image field distortion via field        magnification change in x and y directions by 0.5 parts per        million and by 0.09 ppm respectively, field        orthogonality/rotation change by −0.49 parts per million, and        field rotation by 1.1 parts per million;    -   the edge distribution optimum shown in FIG. 10D was obtained by        minimizing cost function of Eq. (2). The cost function of Eq.        (2), representing distribution standard deviation of edges in        FIG. 10D, was reduced by 86% for vertical edges and by 72% for        horizontal edges relative to the cost function of Eq. (2) for        distribution of corresponding edges in FIG. 10A. The projector        tuning that resulted in improvement of the edge placement shown        in FIG. 10D involved changing the illuminator setup from the one        shown in FIG. 11A to the one shown in FIG. 11B, as well as        tuning of the image field distortion via field magnification        change in x- and y-directions by −0.34 parts per million and by        0.05 parts per million respectively, field        orthogonality/rotation change by 0.143 parts per million, and        field rotation by −0.58 parts per million;

Particularly, in one example shown in FIGS. 10A, 10B, 10C, 10D inreference to FIGS. 11A, 11B, the illuminator setup is adjusted to alterthe illumination layout, and/or polarization of light delivered to themask, and/or illumination flare. Note the difference in the location ofthe bright illuminator pupil segments from which the light illuminationthe reticle emerges (FIG. 11 a shows windmill layout, FIG. 11B showscross layout of the illumination poles). The result of tuning ormodifying the illumination setup from the pattern of FIG. 11A to that ofFIG. 11B, the change in the pattern edge locations was achieved from theones shown in FIGS. 10B, 10C to the ones shown in FIG. 10D. A skilledartisan will readily appreciate that the use of the illuminatordistribution according to that of FIG. 11B minimizes the pattern edgedisplacements and minimizes both of cost functions of Eq. (2) and Eq.(3).

Alternatively or in addition, the numerical aperture of the projectionlens can be modified to alter the lens attributes such as flare,apodization and/or projection pupil aberrations.

In another related implementation, when projector is of the scannertype, the synchronization of scanning of the reticle and scanning of thewafer can be adjusted to modify scan synchronization errors. This latteradjustment will impact the sensitivity of EPE_(i,l) (as defined by Eq.(1)) to the adjustments of field curvature, field distortions andprojection lens aberrations.

Additional or alternative optimization of the cost functions of Eqs. (2)through (4) and the overall edge-placement error is achieved as a resultof modifications introduced to the scanner setup with a proviso thatsuch modifications or tuning may cause the variations of key imagingperformance metrics (such as image depth of focus, DOF; exposurelatitude or EL; and mask error enhancement factor, MEEF). DOF definesallowed imaging defocus range over which pattern CDs meet the tolerancerequirements, EL is the exposure level variation over which the patternCDs meet the tolerance requirements. MEEF is the ratio of imagedimension change to the mask dimension change causing the image change.To deliver the imaging of the mask onto the image plane withpractically-acceptable quality, the DOF and EL must be larger thanminima defined by the integrated device requirements. The DOF and ELminima and MEEF maxima are defined by the imaging process engineer inaccordance with the requirements of given device-manufacturing process.For example, the degree of flatness of a wafer might impose a “minimumDOF” requirement. In practice, therefore, it may be preferred tominimize the image-edge-placement errors by altering the projectorsignature, the imaging setup, the image field curvature and distortionand projection lens aberrations, while simultaneously verifying that foreach new projector signature and imaging setup, the imaging metrics,such as DOF, EL, MEEF, do not violate the imposed constraints. In otherwords, a co-optimization approach involving cost functions (2) through(4) and these additional imaging metrics is required.

By adjusting the scanner imaging setup and signatures that can becarried in real time and combined with the adjustment(s) to projectionlens field curvature, image field distortions and projection lensaberrations, the cost functions of Eqs. (2) through (4) can be optimizedleading to optimized mask-pattern imaging.

It is appreciated, therefore, that embodiments of the inventionfacilitate and enable modifications and tangible adjustments to thehardware of a projection lens and a scanner setup to:

-   -   a) control placement of image edges across various locations in        the image field;    -   b) accomplish desired distribution of image edges at various        locations in the image field;    -   c) minimize image edge displacements at various locations in the        image field;    -   d) minimize the range of image edge displacements at various        locations in the image field;    -   e) accomplish desired imaging performance in terms of image edge        placement, image depth of focus, process latitude, mask        enhancement errors at various locations in the image field under        various imaging conditions;    -   f) match the image dimensions and placement produced by one        imaging tool with the image dimensions and placements of other        tools;    -   g) Control locations and distributions of the image edges across        various locations in the image field;    -   h) Minimize pattern edge displacements that are difficult to        measure directly;    -   i) Co-optimize the edge locations, depth of focus, imaging        latitude, and mask enhancement errors of images under customized        imaging conditions;    -   j) Co-optimize image edge locations, depth of focus, process        latitude, and mask enhancement errors specific to imaging setup        defined by the illumination and projection conditions.

In a related implementation, when projector is of a scanner type, thesynchronization of the scanning of the reticle and the scanning of thewafer (see FIG. 3B) can be also modified or adjusted to reduce scansynchronization errors. This latter adjustment will impact thesensitivity of EPE_(i,l) (as defined by Eq. (1)) to the adjustments offield curvature, field distortions and projection lens aberrations.

As such, implementing the intra-field corrections requires a tuning ofvarious subsystems. The corrections reflected in FIGS. 10A, 10B, 10C,10D were found by analyzing the impact of the projector setup, projectorsignature and projection aberration on paten) placement. This analysiswas part of the optimization sequence involving tuning of the projectorsetup, scanner and aberrations optimize one of the cost functions ofEqs. (2) through (4). As the result of this analysis, optima for theprojection setup, signature and aberrations were found within theprojector adjustment space, determined by the adjustments availablewithin the projection tool (in that sense the optimization process isspecific to a given projection tool).

References throughout this specification to “one embodiment,” “anembodiment,” “a related embodiment,” or similar language mean that aparticular feature, structure, or characteristic described in connectionwith the referred to “embodiment” is included in at least one embodimentof the present invention. Thus, appearances of the phrases “in oneembodiment,” “in an embodiment,” and similar language throughout thisspecification may, but do not necessarily, all refer to the sameembodiment. It is to be understood that no portion of disclosure, takenon its own and in possible connection with a figure, is intended toprovide a complete description of all features of the invention.

Within this specification, embodiments have been described in a way thatenables a clear and concise specification to be written, but it isintended and will be appreciated that embodiments may be variouslycombined or separated without parting from the scope of the invention.In particular, it will be appreciated that each of the featuresdescribed herein is applicable to most if not all aspects of theinvention.

In addition, when the present disclosure describes features of theinvention with reference to corresponding drawings (in which likenumbers represent the same or similar elements, wherever possible), thedepicted structural elements are generally not to scale, for purposes ofemphasis and understanding. It is to be understood that no singledrawing is intended to support a complete description of all features ofthe invention. In other words, a given drawing is generally descriptiveof only some, and not necessarily all, features of the invention. Agiven drawing and an associated portion of the disclosure containing adescription referencing such drawing do not, generally, contain allelements of a particular view or all features that can be presented isthis view, at least for purposes of simplifying the given drawing anddiscussion, and directing the discussion to particular elements that arefeatured in this drawing. A skilled artisan will recognize that theinvention may possibly be practiced without one or more of the specificfeatures, elements, components, structures, details, or characteristics,or with the use of other methods, components, materials, and so forth.Therefore, although a particular detail of an embodiment of theinvention may not be necessarily shown in each and every drawingdescribing such embodiment, the presence of this particular detail inthe drawing may be implied unless the context of the descriptionrequires otherwise. The described single features, structures, orcharacteristics of the invention may be combined in any suitable mannerin one or more further embodiments.

The invention as recited in claims appended to this disclosure isintended to be assessed in light of the disclosure as a whole, includingfeatures disclosed in prior art to which reference is made.

For the purposes of this disclosure and the appended claims, the use ofthe terms “substantially”, “approximately”, “about” and similar terms inreference to a descriptor of a value, element, property orcharacteristic at hand is intended to emphasize that the value, element,property, or characteristic referred to, while not necessarily beingexactly as stated, would nevertheless be considered, for practicalpurposes, as stated by a person of skill in the art. These terms, asapplied to a specified characteristic or quality descriptor means“mostly”, “mainly”, “considerably”, “by and large”, “essentially”, “togreat or significant extent”, “largely but not necessarily wholly thesame” such as to reasonably denote language of approximation anddescribe the specified characteristic or descriptor so that its scopewould be understood by a person of ordinary skill in the art. In onespecific case, the terms “approximately”, “substantially”, and “about”,when used in reference to a numerical value, represent a range of plusor minus 20% with respect to the specified value, more preferably plusor minus 10%, even more preferably plus or minus 5%, most preferablyplus or minus 2% with respect to the specified value. As a non-limitingexample, two values being “substantially equal” to one another impliesthat the difference between the two values may be within the range of+/−20% of the value itself, preferably within the +/−10% range of thevalue itself, more preferably within the range of +/−5% of the valueitself, and even more preferably within the range of +/−2% or less ofthe value itself

The use of these terms in describing a chosen characteristic or conceptneither implies nor provides any basis for indefiniteness and for addinga numerical limitation to the specified characteristic or descriptor. Asunderstood by a skilled artisan, the practical deviation of the exactvalue or characteristic of such value, element, or property from thatstated falls and may vary within a numerical range defined by anexperimental measurement error that is typical when using a measurementmethod accepted in the art for such purposes.

Disclosed aspects, or portions of these aspects, may be combined in waysnot listed above. Changes may be made without departing from the scopeof the invention. In view of the numerous possible embodiments to whichthe principles of the disclosed invention may be applied, the inventionshould not be viewed as being limited to the disclosed example.

1. A method for controlling a placement of an edge of an image of areticle pattern formed on a wafer in a lithographic exposure processwith an exposure tool having an optical projection system, the methodcomprising: determining first operational factors based on operation ofsaid exposure tool, first operational factors representing the exposuretool the first operational factors including i) setup parameters of saidexposure tool; ii) optical aberrations of an illuminator and aprojection lens of the optical projection system, the illuminator andthe projection lens being in optical communication through a reticlecarrying the reticle pattern; and iii) a signature of the optical systemof the exposure tool; optimizing, with a computer device, a meritfunction to produce a value of change of a setup parameter from thesetup parameters, wherein the merit function represents a contributionof a first operational factor, from the first operational factors, to afirst measure of a shift in a position of an edge of said image withrespect to a target position thereof; and modifying, based on said valueof change, the exposure tool by changing at least one of a) one or moreof a presence, position, orientation, size and shape of an opticalcomponent of the optical projection system; and b) a parameter ofscanning synchronization of the exposure tool to form an image with asecond measure of said shift, wherein the second measure is reduce incomparison with the first measure.
 2. A method according to claim 1,wherein the determining includes performing at least one wafer-exposurerun in said exposure tool to identify (i) the setup parameters thatinclude one or more of optical axis tilt, distribution of illuminationin an exit pupil of the illuminator, polarization status in an exitpupil of the projection lens, an illuminator flare, a projection lensflare, a numerical aperture of the projection lens, projection lensaberrations, a shape of the exit pupil of the projection lens, opticalapodization, an optical transmission of the optical projection system,and (ii) scanning synchronization of the exposure tool.
 3. A methodaccording to claim 1, wherein the determining includes performinganalysis of operation of said exposure tool to determine scanningsynchronization errors.
 4. A method according to claim 1, wherein ameasure from the first and second measures is a vector parameter.
 5. Amethod according to claim 1, wherein the determining includes performingat least one wafer-exposure run with said exposure tool to determine,with the use of an optical detector of the exposure tool, said firstoperational factors.
 6. A method according to claim 5, wherein thedetermining includes performing the at least one wafer-exposure run todetermine second operational factors representing an image-overlaycharacteristic of said exposure tool.
 7. A method according to claim 6,wherein the optimizing includes optimizing said merit function based onany of i) a first operational factor from the first operational factors,and ii) a second operational factor from the second operational factors,wherein said optimizing produces an output containing an updated designparameter of the exposure tool, and wherein the modifying includeschanging hardware of the exposure tool to assemble an updated exposuretool having said updated design parameter.
 8. A method according toclaim 1, further comprising defining first relationships among imagedefocus, exposure dose, and critical dimensions of said image at thewafer.
 9. A method according to claim 8, wherein said defining firstrelationships includes defining said first relationships at multipledifferent locations of a scanner slit of said exposure tool.
 10. Amethod according to claim 9, further comprising calculating saidcritical dimensions based on said first relationships, the imagedefocus, and the exposure dose; applying a blur correction based on atleast one of defocus data and scan synchronization data generated bysaid exposure tool; and comparing calculated critical dimensions withtarget critical dimensions corresponding to a design specification. 11.A method for controlling a placement of an edge of an image of an objectpattern, formed on a workpiece in a lithographic exposure process withan exposure tool, the exposure tool having an optical system thatincludes an illuminator and a projection optical system, the methodcomprising: inputting a target position of an edge of a pattern image;computing a cost function of a plurality of variables including aposition of an edge of a pattern image and setup parameters of theexposure tool.
 12. A method according to claim 11, wherein saidcomputing includes computing a cost function based on said plurality ofvariables that includes a parameter of scanning synchronization of theexposure tool.